Ipv6 Cache; Chapter 45 Ipv6 Cache; Overview - ZyXEL Communications VES1724-56 User Manual

This chapter describe the IPv6 caches on the Switch. Refer to
information about IPv6.

45.1 Overview

The Switch uses the Neighbor Discovery Protocol (NDP) to discover other IPv6 devices and track
their reachability in a network. The Switch uses the following ICMPv6 messages types:
• Neighbor solicitation: A request from a host to determine a neighbor's link-layer address (MAC
address) and detect if the neighbor is still reachable. A neighbor being "reachable" means it
responds to a neighbor solicitation message (from the host) with a neighbor advertisement
message.
• Neighbor advertisement: A response from a node to announce its link-layer address.
• Router solicitation: A request from a host to locate a router that can act as the default router and
forward packets.
• Router advertisement: A response to a router solicitation or a periodical multicast advertisement
from a router to advertise its presence and other parameters.
An IPv6 host is required to have a neighbor cache, destination cache, prefix list and default router
list. The Switch maintains and updates its IPv6 caches constantly using the information from
response messages. In IPv6, the Switch configures a link-local address automatically, and then
sends a neighbor solicitation message to check if the address is unique. If there is an address to be
resolved or verified, the Switch also sends out a neighbor solicitation message. When the Switch
receives a neighbor advertisement in response, it stores the neighbor's link-layer address in the
neighbor cache. When the Switch uses a router solicitation message to query for a router and
receives a router advertisement message, it adds the router's information to the neighbor cache,
prefix list and destination cache. The Switch creates an entry in the default router list cache if the
router can be used as a default router.
When the Switch needs to send a packet, it first consults the destination cache to determine the
next hop. If there is no matching entry in the destination cache, the Switch uses the prefix list to
determine whether the destination address is on-link and can be reached directly without passing
through a router. If the address is on-link, the address is considered as the next hop. Otherwise,
the Switch determines the next-hop from the default router list or routing table. Once the next hop
IP address is known, the Switch looks into the neighbor cache to get the link-layer address and
sends the packet when the neighbor is reachable. If the Switch cannot find an entry in the neighbor
cache or the state for the neighbor is not reachable, it starts the address resolution process. This
helps reduce the number of IPv6 solicitation and advertisement messages.
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